In the construction industry, plastic pipe or conduit is becoming increasingly employed for a wide variety of uses such as water or sewer lines, or electrical conduit. In order to install such pipe or conduit in the field it has to be bent, and in order to bend plastic pipe or conduit, such as PVC, it has to be heated to a softening temperature. For proper bending the temperature cannot be too hot and it cannot be too cold. Moreover the temperature should be uniform over the axial length of the segment to be bent.
If the temperature is too hot the pipe or segment cannot be properly handled to be field formed in a jig such as shown in prior U.S. Pat. Nos. 4,156,588 and 4,255,378. Also if the pipe or conduit is too cold or not uniformly heated poor quality bends result.
One form of pipe or conduit heater utilizing resistance elements is sold under the trademark HOTBOX.RTM. by Thermotools Company of Cleveland, Ohio. The heater includes a housing with heating elements along one side. A cover may be opened to position the tube or pipe in the housing on rollers. When the cover is closed the pipe or conduit is rotated, either with a power drive or by hand to provide even heating. This form of heater is rather bulky and the larger models even require wheels.
Another form of heater also sold under the trademark HOTBOX.RTM. by Thermotools of Cleveland, Ohio is that shown in U.S. Pat. No. 3,843,858. This model of heater uses a relatively heavy walled metal pipe which is heated by induction through the use of external windings. The heavy walled pipe acts as a heat sink to obtain a substantially uniform heat range throughout its length. In the commercial form the windings are enclosed by insulation in a tubular housing. The ends of the pipe, being exposed, are more difficult to heat and the temperature tends to drop more at the pipe ends. This metal pipe form of heater has the advantage that the plastic pipe or conduit does not have to be rotated, and, depending on the size, several sections of conduit may be heated simultaneously and left in the heater until required.
This type of heater, while quite effective, is also relatively heavy not only because of the heavy walled metal pipe but also the rather large number of windings of copper wire required. More importantly the heater is difficult and expensive to manufacture because of the care which must be taken with the induction heating copper windings.
It would therefore be desirable to have a heater of the heat sink pipe type which would be of lighter weight, and be more easily fabricated at lower cost.